There are several different types of scanners available to personal computer users, generally classified as either reflective or transparency scanners. Transparency scanners are typically used to digitized 35 millimeter slide or negatives.
Reflective scanner systems process light reflected from the surface of the image media being scanned. Reflected light is collected by a scanning element, typically a charge-coupled device (CCD) array. To obtain digital color information about the image to be scanned, a series of red, blue and green filters are sequentially, placed between the image and the scanning element. A similar technique is also used in color television systems and the like.
In transparency scanner systems, light is directed through the image and collected by a scanning element the same as or similar to that used in reflective systems. Color information about the image being scanned is also obtained in the same way as in reflective systems.
Overall spacial resolutions of transparency scanners are similar to full page scanners. Thus, an 8".times.11" page, digitized at 300 DPI, is approximately equivalent to a transparency scanner digitizing at 2,500 dots per inch. Color resolutions typically range between 15 bits to 32 bits per pixel. However, 24 bits per pixel color resolution is rarely achievable in present systems.
Most existing color transparency scanners must make three passes to digitize red, green and blue information. See for example U.S. Pat. No. 4,907,280. The three colored images, also referred to as color planes, must then be aligned properly to produce good quality output data. Without an expensive transparency transport system which is omitted in low-cost scanner systems, the color planes are usually, at least slightly, misaligned owing to reference sensor drift and poor alignment tolerances. Such misalignment translates into low fidelity reproduction of the original image.
Low- to mid-priced scanners currently available in the marketplace typically use a CCD with a signal-to-noise ratio greater than 256. Such a CCD allows an 8-bit binary number to represent the digitized information for each color. Because of inherent noise and quantization errors, the dynamic range of such scanners is limited to only 6 bits of useful information which correspond to 64 levels of quantization, i.e. shades. Resolution is further degraded by the nonlinearity of the CCD, especially over the range from very brightly to very dimly illuminated images. By way of comparison, the dynamic range of film can reproduce 4,000 shades.
In high-priced scanners, dynamic range is increased by using an expensive analog-to-digital (A/D) converter and associated signal conditioning electronics. Mid-priced models occasionally provide a larger dynamic range by scanning the original several times at different light intensities, which increases the scan/processing time and decreases the accuracy of color plane registration.
A single pass scanner digitizes each line of light data, collected in a single pass, three times, one digitization for each color, namely red, blue and green, before advancing to the next line. Thus, the colored filters or light sources are switched three times per line before moving to the next line. Image data is organized into a packed format which requires all of the three color components for each pixel to be stored contiguously. Thus, the scanner or computer only has to process one line of data at a time before storing or displaying that portion of the image. Consequently, single pass scanners provide a more easily displayable interim data format, and therefore require less CPU overhead.
In the prior art of scanners pertinent to the present invention, U.S. Pat. No. 4,937,663 (Gerlach) teaches an apparatus for scanning and recording a color image having a scanning element with scanning periods equal to the reciprocal of the flicker rate of a light source illuminating the color image. A rotating multi-segment color filter is rotated at a rate such that each filter segment is interposed in the image light path for a period of time greater than the reciprocal of the flicker rate.
Gerlach processes red, green and blue information for each line before indexing to the next line. The rotation rate of the color filter wheel allows each filter to expose the CCD for 11 milliseconds. During the first 8.33 milliseconds, the color information is collected by the CCD. During the remaining 2.67 milliseconds, the collected information is clocked out of the analog shift registers of a CCD.
It should be noted that Gerlach utilizes a current source powered by 60 Hz AC line voltage for the light source of his system. It also should be noted that Gerlach disposes his multi-segment filter wheel between the image light focusing subsystem and the scanning element of his system.